Crusty White Stuff on Pots
This powdery material is the remains of fertilizer or minerals in the potting soil inside the pot. Water wicks through the clay pot wall and the minerals dissolved in it dry on the outer surface.
The white stuff is composed of calcium, magnesium, sulphate, nitrate, phosphate and ammonium salts.
Simply soak the pot for an hour in a bucket of water and scrub off the white powder.